Cellular response to light, drugs, neurotransmitters and hormones is often mediated by G protein coupled receptors (GPCRs) and their associated G proteins. Misregulation of G protein signaling is the cause of diseases ranging from heart disease to depression, and about half of all prescription drugs sold in the U.S. target G protein signaling components. Typically signaling is initiated when a ligand binds to a GPCR and causes the Galpha protein to exchange its bound GDP for GTP and release the Gbeta/gamma subunits. Signaling is terminated by the intrinsic GTPase activity of the Galpha subunit, an activity that is accelerated by RGS (regulators of G protein signaling) proteins. Recently our group of collaborators identified Arabidopsis signaling components that differ from typical G proteins in two important ways: (1) the plant D- glucose receptor is composed of a ligand-binding receptor domain fused to an active RGS domain and (2) the associated plant Galpha protein is constitutively GTP bound. In contrast to typical GPCRs , which promote nucleotide exchange, we hypothesize that this GPCR pathway entails the atypical mechanism of ligand-regulated signal termination. This proposal outlines combined genetic and biochemical approaches to characterize the early steps in plant glucose sensing. My aims are to (1) determine if the plant GPCR /RGS hybrid has glucose-dependent GTPase accelerating activity and/or if it affects GDP/GTP exchange (2) determine the role of G protein signaling in regulating production of the phophatidic acid second messenger in vivo, and (3) use the atypical plant system as a model to identify structural determinants of Galpha activation and inactivation. A majority of prescription drugs sold in the U.S. target cell surface receptors and downstream G protein signaling pathways, and these drugs help manage problems ranging from heart disease to depression. A group of factors called RGS proteins mediate desensitization to these drugs, and the interaction between receptors and RGS proteins may represent a new and relatively unexplored drug target. This proposal aims to characterize how the interaction between receptors and RGS proteins contributes to signal termination. [unreadable] [unreadable] [unreadable]